My advices:
- Install all windows essential updates (SP2 recommended) and enable Automatic updates
- Disable unnecessary Windows services
- Install at least one spyware tracking/preventing tool (Spywareblaster and Spybot SD-Resident Tea Timer, Ad-aware. Three won’t conflict each other)
- Install Avast Antivirus (if Home version, use a script blocker)
- Enable SP2 Firewall and install a third party software firewall (Agnitum Outpost, ZoneAlarm or Sygate. Only one!). If you can, have a hardware firewall (router)
- Install a spamkiller (Spamihilator ;))
As you can see, it’s not only a matter of antivirus ;D